Method of charging a zinc oxide photoconductive layer with a positive charge



Nov. 19, 1968 E. METHOD OF CHARGING C. GIAIMO, JR

A ZINC OXIDE PHOTOCONDUCTIVE LAYER WITH A POSITIVE CHARGE Filed Dec.

Varn- 400- dead F18 INVENTOR. flan/4R0 (I GuwqJr- Attorney United States Patent 3,412,242 METHOD OF CHARGING A ZINC OXIDE PHOTOCONDUCTIVE LAYER WITH A POSITIVE CHARGE Edward C. Giaimo, Jr., Princeton, N .J., assignor to Radio Corporation of America, a corporation of Delaware Filed Dec. 10, 1965, Ser. No. 512,904 4 Claims. (Cl. 250-495) ABSTRACT OF THE DISCLOSURE A method of charging a zinc oxide layer with a useable positive charge comprises the steps of: (1) directing a negative corona discharge to the layer to charge it negatively; and (2) then directing a positive corona discharge to the negatively charged layer to change the charge from negative to positive.

This invention relates generally to electrophotography, and more particularly to an improved method of charging an electrophotographic recording element in an electrophotographic process. The improved method is particularly useful for charging a recording element, of the type having a photoconductive layer of zinc oxide, with a positive electrostatic charge of sufficient amplitude to be useful in a conventional electrophotographic process.

In an electrophotographic process of the type described in RCA Review, volume XV, p. 469, December 1954, for example, an electrophotographic recording element, comprising a layer of photoconductive zinc oxide dispersed in a resin binder on a backing of suitable conductivity, is charged negatively so that the photoconductive layer carries a fairly uniform negative electrostatic charge over its surface. The recording element is then exposed to a light image to discharge the surface in accordance with the intensity of light falling on it and to leave an electrostatic latent image thereon. The latent image is then developed by applying a finely divided electrostatically attractable material, such as an electroscopic mixture of carbon particles and iron filings, thereon. The developed image may be either fixed to the recording element, as by applying heat to the toner particles to fuse them, or the unfixed developed image may be transferred to another surface.

The electrophotographic recording element may be charged by any suitable method known in the art, as, for example, by the corona charging method described in the aforementioned issue of RCA Review.

Although an electrophotographic recording element having a photoconductive layer comprising zinc oxide can be charged negatively quite easily, it cannot be charged positively with a suitably useable charge by prior art, conventional charging methods. Even if the photoconductive zinc oxide layer is thoroughly dark adapted, that is, stored in the dark and completely unaffected by light, the maximum positive charge retained by the layer is much smaller, and the time constant of decay of the charge is much shorter than for negatively charged layers. A positive electrostatic charge on a photoconductive zinc oxide layer of a recording element, however, is very desirable in certain electrostatic printing processes, as, for example, where a positively charged photoconductive layer is exposed by light from a phosphor screen of a cathode ray tube to form a latent image, and where the latent image is developed by a developer of positive toner particles because the need for reversal-type developers is obviated.

It is a general object of the present invention to provide an improved method for charging a recording element.

Another object of the present invention is to provide an improved method of charging positively the surface of a photoconductive zinc oxide layer of an electrophotographic recording element.

Briefly, the improved method of charging an electrophotographic recording element comprises, first, charging the photoconducting surface of the recording element with a charge of one polarity, and then charging the surface with a charge of the opposite polarity. Thus, the surface of a photoconducting zinc oxide layer, which ordinarily cannot be charged sufiiciently positive for practical use in an electrophotographic process by prior art charging methods, can be charged adequately positive by first directing a negative corona discharge upon the surface, and then following with a positive corona discharge directed upon the surface.

The invention will now be described in detail with reference to the accompanying drawing, in which:

FIG. 1 is a graph of the positive voltage and its dark decay time accepted by a photoconductive zinc oxide layer by methods of the prior art; and

FIG. 2 is a graph of voltages of different polarities applied by the improved method across a recording element, having a zinc oxide photoconductive layer, vs. the dark decay time of the voltages.

Referring now to FIG. 1, there is shown a curve 10 of the positive charge acceptance of, and dark decay rate of the charge on, a photoconductive zinc oxide layer on a conductive backing. The ordinate 12 at the time of zero second (FIG. 1) represents a positive voltage of about 50 volts measured across the zinc oxide layer, the back of the layer being at zero potential, after the surface of the layer was exposed to an initial positive corona discharge of about 7 kilovolts for about 5 seconds. Voltage measurements across the recording element may be made by an electrometer of the type described in RCA Review, vol. XXlL'No. 4, p. 780, December 1961. The continuous portion 14 of the curve 10, from zero to 5 seconds, represents the dark decay of the positive charge over a period of 5 seconds. Thus, an initial positive charge of about 50 volts across the photoconductive zinc oxide layer decays rapidly to zero in 5 seconds, a duration too short for practical use in conventional electrophotographic processes. Actually, it is not possible to charge an uncharged photoconductive zinc oxide layer positively to a practical amplitude by employing only a positive corona discharge for any time duration.

When the surface of the photoconductive zinc oxide layer is exposed to a negative corona discharge of about 7 kilovolts for about 5 seconds, a voltage of about 500 volts is measured across the layer, the back of the layer being at zero potential, as illustrated by the ordinate 16 of the curve 17 in FIG. 2. After a period of five seconds, from zero to 5 seconds on the curve 17, this negative charge dark decays at a rate represented by the continuous portion 18 of the curve 17. Thus, after discontinuing the negative corona discharge for 5 seconds the voltage across the photoconductive zinc oxide layer decays from about 500 volts to about -4S0 volts.

If the photoconductive zinc oxide layer is now exposed to a positive corona discharge of about 7 kilovolts for about 5 seconds, the layer is charged to a positive voltage of about 275 volts, as illustrated by the ordinate 20 of the curve 17. This positive voltage dark decays relatively slowly, as illustrated by the portion 22 of the curve 17, for a sufficiently long time to enable the positively charged photoconductive zinc oxide layer to be exposed and developed in a conventional electrophotographic process. Thus, it is seen that whereas a photoconductive layer of zinc oxide cannot .becharged initiallyto avoltage greater- :ticles; the improvementof charging-said photoconductive than about 50 volts by prior art methods, it can be charged to about 275 volts by a positive corona discharge if the layer is first charged with a corona discharge of negative polarity.

From theforegoing description, it will be apparent that there has been provided an improved method of charging positively a recording element that would not ordinarily accept a practical positive charge by prior art charging methods. While the improved method has been illustrated for electrostatically charging a photoconductive zinc oxide layer, variations in the method and applications thereof coming within the spirit of this invention will, no doubt, suggest themselves to those skilled in the art. For example, negative and positive corona discharges of as little as 5 kilovolts with respect to ground may be used sequentially for a duration of only one second each to obtain satisfactory positive charging of certain dye sensitized zinc oxide recording elements. Hence, it is desired that the foregoing shall be considered merely as illustrative and not in a limiting sense.

What is claimed is:

1. In an electrophotographic process wherein a charged insulating photoconductive layer comprising zinc oxide on a conductive backing is exposed to a light image to form a latent electrostatic image thereon, and said latent electrostatic image is developed by electroscopic toner parlayer positively comprising;

first charging said photoconductive layer with a charge of negative polarity, and

immediately thereafter charging said photoconductive layer with a charge of positive polarity.

2. A method of ;charging the surface of a photoconductive layer comprising zinc oxide with a positive charge, said 'method'comprising the steps of:

first directing a negative corona discharge to said surface to charge said surface negatively, and immediately thereafter directing a positive corona discharge to said surface to neutralize the negative charge thereon and to leave a positive charge thereon.

3. The method of claimZ wherein each of said charging steps is for a duration of at least one second.

4. The method of claim 2 wherein each of said corona discharges is at a voltage of at least 5,000 volts and ap' plied for a duration or" at least one second. References Cited UNITED STATES PATENTS 2,965,481 12/1960 Gundlach 250--49.5 3,147,415 9/1964 Oliphant 317-262 RALPH G. NILSON, Primary Examiner. A. L. BIRCH, Assistant Examiner. 

